Emulsified perfluorochemicals provide an effective means of transporting oxygen in biological systems under a variety of conditions. Combined with hydroxyethyl starch and electrolytes these dispersed compounds can be used for partial and total blood replacement in vivo. Such preparations must be completely nontoxic and possess physical characteristics that are within fairly well defined limits. Viscosity, oncotic pressure, and osmotic pressure are especially important. All components must be shown to be nontoxic before incorporation into these products. Bloodless animals obtained with such preparations are ideal models with many biological applications. They also can be used for long-term organ perfusion in vitro at 37 degrees centigrade. Liver so perfused has very high bile output, and the QO2 of the liver after 18 to 24 hours of perfusion approaches that of normal liver. A variety of new perfluorochemicals are becoming available for study and preparations even better than those now on hand should be possible.